In recent years, it has become desirable for printed wiring boards to include a substrate with a good heat dissipation property, as electronic components are mounted in higher density. A metal core substrate has been known as a printed wiring board excellent in heat dissipation property, and has already been put to practical use. The metal core substrate uses a metal having high thermal conductivity, such as aluminum (Al), copper (Cu), or the like, as a core material, and thereby can dissipate heat from a heat-generating component throughout the printed wiring board and suppress an increase in the temperature of the heat-generating component. Above all, aluminum, which has a low specific gravity, is generally used as a core material.
However, aluminum has a high coefficient of thermal expansion of about 24 ppm/° C., whereas a ceramic component has a low coefficient of thermal expansion of about 7 ppm/° C. Therefore, there arises a problem that when a heat cycle test is conducted, a crack occurs at a solder joint portion due to a difference in the coefficients of thermal expansion of aluminum and the ceramic component, failing to achieve mounting reliability.
As a core material capable of solving the above problem, carbon fiber reinforced plastic (hereinafter also referred to as CFRP) has been known (for example, see Japanese Patent Laying-Open No. 11-40902 (Patent Document 1)). CFRP is a composite material including carbon fiber and resin. The carbon fiber is roughly classified into PAN (polyacrylonitrile)-based carbon fiber and pitch-based carbon fiber (carbon fiber using petroleum pitch as a starting material). The carbon fiber, both PAN-based and pitch-based, has a low coefficient of thermal expansion of ±2 ppm/° C. In the carbon fiber, pitch-based carbon fiber has a thermal conductivity higher than that of PAN-based carbon fiber, and some of pitch-based fibers have a thermal conductivity of not less than 500 W/(m·K). In addition, the carbon fiber has a low specific gravity of about 2 g/cm3. As a CFRP core, a core formed by laminating carbon fibers each arranged unidirectionally with the directions thereof being changed, or impregnating cloth with resin is used. If a core substrate can be fabricated using this CFRP, a substrate with high thermal conductivity and more excellent in mounting reliability than aluminum can be obtained.
Since the core materials described above are all electrically conductive, it is necessary to insulate the core material from a penetrating through hole for connecting interconnections provided above and below the core material, using resin for filling a through hole. Generally, a through hole is filled by stacking and laminating a semi-cured prepreg, which is prepared by impregnating glass cloth with epoxy resin, above and below a core material having the through hole formed therein. That is, the prepreg is melted by heat at the time of lamination, and the resin caused to flow by pressurization is charged into the through hole.
However, in the case where a conventional prepreg is used to fill a through hole, since resin has a low thermal conductivity of about 0.2 W/(m·K) heat dissipation property is limited by the hole-filling resin, causing a problem that heat from a heat-generating component cannot be fully conducted to the core. Further, since the charged resin has a coefficient of thermal expansion of 60 ppm/° C. whereas the CFRP core has a coefficient of thermal expansion of about 0 ppm/° C., there is a large difference in the coefficients of thermal expansion, leading to occurrence of a crack in the resin or the CFRP core.
Therefore, the inventors of the present invention found that a CFRP core substrate having higher heat dissipation property and higher reliability than a conventional one can be obtained by applying a filler-containing resin sheet having a high thermal conductivity and a low coefficient of thermal expansion as a hole-filling resin, and filling a hole by laminating the resin sheet (for example, see Japanese Patent Laying-Open No. 2008-53362 (Patent Document 2)).